In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In the basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Such light-emitting elements are self-luminous elements and have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for backlights, for example; thus, such light-emitting elements are thought to be suitable for flat panel display elements. Displays including such light-emitting elements are also highly advantageous in that they can be thin and lightweight. Furthermore, very high speed response is one of the features of such elements.
Since light-emitting layers of such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission. Therefore, large-area elements can be easily formed. This is a feature difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting elements also have great potential as planar light sources applicable to lightings and the like.
In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is put in an excited state to provide light emission.
It is to be noted that the excited states formed by an organic compound include a singlet excited state and a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, whereas luminescence from the triplet excited state (T*) is referred to as phosphorescence. In addition, the statistical generation ratio thereof in the light-emitting element is considered to be as follows: S*:T*=1:3.
In a compound that emits light from the singlet excited state (hereinafter, referred to as a fluorescent compound), at room temperature, generally light emission from the triplet excited state (phosphorescence) is not observed while only light emission from the singlet excited state (fluorescence) is observed. Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on the ratio of S* to T* which is 1:3.
In contrast, in a compound that emits light from the triplet excited state (hereinafter, referred to as a phosphorescent compound), light emission from the triplet excited state (phosphorescence) is observed. Further, in a phosphorescent compound, since intersystem crossing (i.e., transfer from a singlet excited state to a triplet excited state) easily occurs, the internal quantum efficiency can be increased to 100% in theory. That is, higher emission efficiency can be achieved than using a fluorescent compound. For this reason, light-emitting elements using phosphorescent compounds are now under active development in order to obtain highly efficient light-emitting elements.
A white light-emitting element disclosed in Patent Document 1 includes a light-emitting region containing a plurality of kinds of light-emitting dopants which emit phosphorescence.